In a vehicle with an automatic transmission, a torque transferring apparatus is used to transfer torque from a drive unit, such as the vehicle engine, to the vehicle transmission. The torque transferring apparatus is typically interposed between the drive unit and the transmission. Fluid couplings, such as torque converters, are widely employed for this purpose.
The typical torque converter has a torque input member (commonly designated as the pump or impeller) and a torque output member (commonly designated as the turbine). A reaction member (commonly designated as the stator) may be interposed between the pump and the turbine to effect a more favorable direction for the flow of hydraulic fluid exiting the turbine and returning to the torque converter pump.
Hydraulic fluid is supplied to the torque converter under pressure by a fluid supply and valve arrangement. Dynamic circulation of the hydraulic fluid through the torque converter effects rotation of the turbine in response to rotation of the torque converter pump.
Typically, the torque converter pump is coupled to the crankshaft of the vehicle engine, while the turbine is connected to an output shaft which exits the torque converter to serve as the input shaft of the vehicle transmission gear assembly.
When a vehicle is not moving, and while the engine is idling (such as when the driver applies the brake, or the transmission is in neutral or park), the torque converter pump is generally not spinning at a sufficient angular velocity to supply the energy necessary to overcome the static inertia of the vehicle. In this situation, the hydraulic fluid simply flows through the turbine, and ideally the turbine does not rotate. This allows the vehicle to remain at rest, even if the transmission has been shifted into a selected drive range (i.e., a forward gear or reverse) and the engine is running.
As a request for vehicle movement is received (such as when the driver releases the brake, applies the accelerator pedal, or shifts into a forward gear), the rotational speed of the engine, and therefore the rotational speed of the torque converter pump, increases. At some rotational speed of the engine, sufficient energy is imparted to the turbine so that it overcomes the static inertia that had previously prevented the vehicle from moving. At that time, the energy transferred from the torque converter pump to the turbine is delivered to the drive wheels through the transmission.
Fluid exits the turbine and reenters the torque converter pump without any redirection, unless an intermediary such as a stator is interposed in the path which the hydraulic fluid follows between its exit from the turbine and its re-entry into the pump. The stator redirects the hydraulic fluid which has exited the turbine so that the fluid will enter the input of the pump in a direction that will cause the fluid to assist the engine in turning the pump. The force imparted by the returning hydraulic fluid to the pump comprises an additional source of kinetic energy. This additional energy applied to the pump results in an increase in the force applied to drive the turbine, providing torque multiplication.
In operation, torque converters require a source of pressurized hydraulic fluid. The hydraulic fluid is generally drawn from the transmission pan or from a sump, and delivered to the torque converter at a predetermined pressure. A valve controls the pressure of the hydraulic fluid supplied to the torque converter. The pressurized hydraulic fluid is supplied to the torque converter, where it is used to effect a hydraulic torque transfer between the pump and the turbine within the torque converter. Thereafter, the fluid is directed through a cooling system to the fluid supply and then recycled.
Torque converter control valves usually permit fluid flow to the torque converter after the pressure at the outlet of the fluid supply reaches a predetermined value. As the pressure continues to increase, the torque converter valve directs excess fluid to the inlet side of the pump.
Torque converters are often provided with a clutch assembly that effectively locks the torque converter pump and the turbine into a unitary rotating mass under certain operating conditions, for example, when “slip” (i.e., a difference in rotational speed) between the pump and the turbine is not required. Typically, the torque converter clutch or “lockup” clutch is activated to effect unitary rotation of the torque converter pump and turbine in response to reduced hydraulic pressure within the torque converter.
The torque converter clutch assembly may also include a pump clutch, which is operable to disconnect the torque converter pump from the vehicle drive unit. This may be desirable when the vehicle is idling, for example. Embodiments of a torque converter having a clutch assembly including a torque converter clutch and a pump clutch are described in Hemphill et al., U.S. Patent Application Publication No. US 2007-0074943.